I just uninstalled XCOM2, as I finished playing it. When I reloaded GOG Galaxy, it showed that none of the games I previously had installed were there. They still show up as my having purchased them (thankfully!) but they are nowhere to be found onm the hard drive.

I checked the Games directory in my GOG Galaxy folder, and it was deleted.
Installing a new game recreates the Games directory.

Is this a problem with GOG, or with the XCOM2 uninstaller? I'd experiment with it again, but XCOM2 and expansions are about a 66 GB download...

First time i hear of something like this happening and i never experienced something similar no matter if uninstalling a game via Galaxy, the game via its uninstaller, by just directly deleting some games by deleting their folder or even by uninstalling/reinstalling galaxy itself. I will "quickly" download xcom 2 and uninstall it to see if i can recreate your issue (kinda doubt it though as it seems like a really odd occurence). Will get back to you when it's done.

Could not reproduce it. Xcom 2 Uninstall left my other Folders within the Galaxy Folder untouched as expected.
What exactly did you do regarding the uninstall?
And which version of Galaxy you using? (you can see it when you open settings on the bottom left)
I use 2.0.23.4 Beta

Simply put, GOG is awful. Thje same thing just happened to me after I installed Windows. I linked the install path to where I had them installed previously, clicked install and it proceeded to download Cyberpunk again = old files deleted.

I have extremely slow internet and will not be downloading it again.

This is the end of GOG for me. I will download Witcher 3 on steam instead and forget this pile of crap.